Conventional integrated circuits are made up a number of individual devices that are formed in a common plane and interconnected together by one or more layers of metal. Interconnections limit the area and the speed of circuits due to organization of devices in a single plane of a semiconductor wafer such as silicon and due to the longer interconnect lengths that the signal must propagate. Both circuit performance and density can be improved by forming circuits in three dimensions. A three-dimensional structure for an integrated circuit gives a circuit designer a degree of freedom that can lead to significant area reduction and improvement in functionality and speed through preferential reduction of important or critical interconnect lengths. The implications of three dimensional integrated circuits are at short scales, such as within a cell (static random-access memory(SRAM), dynamic random-access memory(DRAM), repeated logic blocks, etc.) as well as between functions (logic to memory functional units, logic to analog functional units, etc.). Moreover, a three dimensional structure offers processor and logic designers the potential for adding more input/output contacts to a given device. As processor and logic devices become more complex and as analog and even power devices are integrated with small signal, digital devices, there is a pent up demand for additional ways to created small packages with added input/output contacts.
Others have used a variety of techniques to provide three-dimensional structures. U.S. Pat. Nos. 5,915,167 and 5,798,297. Still others have provided stacked silicon-on-insulator (SOI) devices. U.S. Pat. Nos. 6,093,623 and 5,426,072. Process techniques are known for bonding together semiconductor wafers. U.S. Pat. Nos. 5,514,235 and 5,849,627. Process techniques are known for cleaving thin semiconductor layers from bulk semiconductor wafers. U.S. Pat. Nos. 6,120,597, 5,909,627, and 6,013,563. Damascene process techniques are known for forming interconnect structures on integrated circuits. U.S. Pat. Nos. 6,157,081; 5,998,292, and 5,614,765, the disclosures of all of the cited patents are herein incorporated by reference.
The invention provides a structure and a method of fabricating three dimensional integrated circuits. The invention may be used to fabricate memory, microprocessor, digital signal processors, logic, linear, power, application specific devices, and combinations thereof.
The method of the invention provides a combination of steps for exfoliating a thin layer from a donor semiconductor substrate, typically silicon and bonding the exfoliated layer to a receiver substrate, also typically silicon, to form a stack of SOI devices that are interconnected. The method forms desired circuits in the receiver and donor substrates, bonds the donor substrate to the receiver, and exfoliates most of the donor substrate. The donor layer has one surface treated to be atomically smooth. It is covered with a protective oxide layer. Exfoliating ion of two species are implanted into through the protected surface and into the donor substrate. The ions are selected from a group of known exfoliant gases including hydrogen and helium. After implant, the substrates are bonded together. In one method of bonding, the receiver substrate is also covered with a protective oxide layer. The bonding operation forms an oxide/oxide bond. However, those skilled in the art could know that one can form silicon/silicon oxide bonds or silicon/silicon bonds. The bonded substrates together to form a bonded structure. It is heated until a portion of the donor substrate exfoliates from the bonded structure and leaves a residual portion of the donor substrate bonded to the receiver substrate.
The residual donor substrate is cleaned, polished, and oxidized. The oxidation is carried out during a low temperature isothermic process or during a rapid thermal annealing process. The oxidation step not only oxidizes the exposed surface of the donor layer but also oxidizes the buried, subcutaneous surface of the donor substrate. This provides a desired insulating layer for SOI devices that are later formed in the donor layer. The outer layer of oxide is stripped to expose the donor surface for forming devices thereon.
In one embodiment the substrates have an intervening insulation layer. That layer includes an interconnect structure that connects devices on one substrate to devices on the other substrate. Conductive plugs or other interconnections are formed in the insulating layer. The plugs are doped polysilicon or a refractory metal alloy, or compound such as tungsten and titanium nitride or a combination of alloy thereof.